If I remember it is The Galactic Empire. Plus, it is a good plot to have wiped 3PO's memory anyways. Granted, Artoo would tell him about his past, but not the complete story from what we know about how Artoo acts. And, even if Artoo is given the storage of 3PO's memory, he would have toyed around with it.

Thing is, 3PO got mindwiped is cause of his loudmouth nature. If he would be the nice quiet type he would be ok.